The __set_name__ method for descriptors

Descriptors generally have to interact with attributes of the managed object, and this is done by inspecting __dict__ on that object (or calling getattr/setattr, but the problem is the same), and finding the key under the specific name.

For this rea­son, the de­scrip­tor will have to know the name of the key to look ­for, which is re­lat­ed to the name of the at­tribute is man­ag­ing.

On pre­vi­ous ver­sions of Python this had to be done ex­plic­it­ly. If we want­ed to ­work around it, there were some more ad­vanced ways to do so. Luck­i­ly, af­ter PEP-487 (added in Python 3.6), there are some en­hance­ments re­gard­ing class cre­ation, which al­so af­fect­s de­scrip­tors.

Let’s re­view the prob­lem, the pre­vi­ous ap­proach­es to tack­le it, and the mod­ern way of solv­ing it.

Configure the name of the descriptor

The de­scrip­tor needs to some­how know which at­tribute will be mod­i­fy­ing, and for this, the most com­mon so­lu­tion is to store the at­tribute name in­ter­nal­ly. For ex­am­ple in:

class LoggedAttr:
    def __init__(self, name=None):
        self.name = name

    def __get__(self, instance, owner):
        if instance is None:
            return self
        return instance.__dict__[self.name]


class Managed:
    descriptor = LoggedAttr('descriptor')

What we re­quire is to check that the name is passed to the de­scrip­tor prop­er­ly, ba­si­cal­ly:

assert Managed.descriptor.name == 'descriptor'

But we don’t want to pass the string 'descriptor' as a parameter when constructing it, because it’s repetitive. Instead, we want this to be configured automatically. Let’s see some options.

A class decorator

With a class dec­o­ra­tor, we could de­fine all dec­o­ra­tors for the class as ­pa­ram­e­ters of the dec­o­ra­tor, and make the as­sign­ment of the name in it as well.

Some­thing like this:

class configure_descriptors:
    def __init__(self, **kwargs):
        self.descs = {dname: dcls(dname) for dname, dcls in kwargs.items()}

    def __call__(self, class_):
        for dname, descriptor in self.descs.items():
            setattr(class_, dname, descriptor)
        return class_


@configure_descriptors(
    descriptor=LoggedAttr
)
class DecoratedManaged:
    """The descriptor is provided by the decorator"""

The con­di­tion is pre­served:

assert DecoratedManaged.descriptor.name == 'descriptor'

In this decorator, we provide the name and the class of the descriptor to be created, and the decorator instantiates the class with this name. We could also have created the instance directly in the descriptor, and then update the value with setattr(descriptor, 'name', dname), which is more general, in case you want to create descriptors that take multiple arguments on their __init__ method, but for this case it’s just fine.

Then we set the new de­scrip­tor (the one that has the name al­ready up­dat­ed on it), to the wrapped class.

How­ev­er, it still seems a bit un­fa­mil­iar or coun­ter-in­tu­itive that we’re defin­ing the de­scrip­tor not in the body of the class, but as a pa­ram­e­ter of a dec­o­ra­tor.

There must be an­oth­er way.

A meta-class

Imagine we flag the class by adding a __set_name = True attribute on it, in order to hint the meta-class that this is going to be one of the attributes that need its name changed. Then the meta-class would look something like:

class MetaDescriptor(type):
    def __new__(cls, clsname, bases, cls_kwargs):
        for attrname, cls_attr in cls_kwargs.items():
            mangled_attr = "_{0}__set_name".format(cls_attr.__class__.__name__)
            if hasattr(cls_attr, mangled_attr):
                setattr(cls_attr, 'name', attrname)
        return super().__new__(cls, clsname, bases, cls_kwargs)


class MetaManaged(metaclass=MetaDescriptor):
    descriptor = LoggedAttr()

And again:

assert MetaManaged.descriptor.name == 'descriptor'

One detail is that the __init__ of the descriptor accepts the name to be nullable so this works. Another option would have been defining only the descriptor assigned to the class, and then, re-mapping the attribute with the instance, passing the name when it’s being constructed on the meta-class. Both options are the same, and the example was made with simplicity in mind.

This works but it has a cou­ple of is­sues. First we have to some­how iden­ti­fy when the class at­tribute needs to be up­dat­ed (in this case, a flag was added to it, but oth­er al­ter­na­tives are no bet­ter at al­l). The sec­ond prob­lem should be rather ob­vi­ous: it’s not a good use of meta-­class­es, and this is overkill (to say the least) for what should be a sim­ple task.

There must be a bet­ter way.

__set_name__

And there is. At least for Python 3.6 and higher. The __set_name__ method was included, which is automatically called when the class is being created, and it receives two parameters: the class and the name of the attribute as it appears defined in the class.

With this, the prob­lem is re­duced to just sim­ply:

class LoggedAttr:
    ...
    def __set_name__(self, owner, name):
        self.name = name

And that’s it, no oth­er code is need­ed. The so­lu­tion is much sim­pler, and it en­tails less prob­lem­s.

Actually, I deliberately named the flag __set_name, to get an idea of what’s coming, and to hint that with __set_name__, Python must be doing something similar to the example, but in this case we shouldn’t worry about it.

Conclusion

Even though it’s fine to just know about the last method, and we could sim­ply use that, it’s still im­por­tant to have fol­lowed this path, think­ing about how things were done pre­vi­ous­ly, be­cause it’s not fair to just as­sume things were al­ways good, and take that for grant­ed. Oth­er­wise, we would miss the evo­lu­tion of the lan­guage, and as­sume there were nev­er is­sues, prob­lems or things that need­ed re­vi­sion.

And more importantly, there still are. Python still has lots of other areas for improvement. Just as in this example __set_name__ seems to solve a small, yet annoying problem, there are many other scenarios on which things are not crystal clear in Python, so the language still needs to evolve.